Rollup merge of #127758 - Zalathar:expression-used, r=oli-obk

coverage: Restrict `ExpressionUsed` simplification to `Code` mappings

In the future, branch and MC/DC mappings might have expressions that don't correspond to any single point in the control-flow graph. That makes it trickier to keep track of which expressions should expect an `ExpressionUsed` node.

We therefore sidestep that complexity by only performing `ExpressionUsed` simplification for expressions associated directly with ordinary `Code` mappings.

(This simplification step is inherited from the original coverage implementation, which only supported `Code` mappings anyway, so there's no particular reason to extend it to other kinds of mappings unless we specifically choose to.)

Relevant to:
- #124154
- #126677
- #124278

```@rustbot``` label +A-code-coverage

compiler/rustc_middle/src/mir/coverage.rs

@@ -220,19 +220,6 @@ pub enum MappingKind {
 }
 
 impl MappingKind {
-    /// Iterator over all coverage terms in this mapping kind.
-    pub fn terms(&self) -> impl Iterator<Item = CovTerm> {
-        let zero = || None.into_iter().chain(None);
-        let one = |a| Some(a).into_iter().chain(None);
-        let two = |a, b| Some(a).into_iter().chain(Some(b));
-        match *self {
-            Self::Code(term) => one(term),
-            Self::Branch { true_term, false_term } => two(true_term, false_term),
-            Self::MCDCBranch { true_term, false_term, .. } => two(true_term, false_term),
-            Self::MCDCDecision(_) => zero(),
-        }
-    }
-
     /// Returns a copy of this mapping kind, in which all coverage terms have
     /// been replaced with ones returned by the given function.
     pub fn map_terms(&self, map_fn: impl Fn(CovTerm) -> CovTerm) -> Self {